Apparatuses And Methods For Coupling A Signal To And/Or From A Cable

ABSTRACT

Apparatuses and methods for coupling a signal to and from a twisted pair cable by non-contact coupling with twisted pairs in the twisted pair cable, such that the signal propagates along the cable between at least two of the twisted pairs. In particular, a coupling unit for coupling a voltage signal to and/or from such a cable, the coupling unit having a first electrode and a second electrode. The electrodes may be electrically isolated from a voltage signal generator and/or a voltage signal processor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/420,787, filed Apr. 8, 2009 which claims benefit of priority to GreatBritain Patent Application Number 0905361.2 filed 27 Mar. 2009, which isincorporated herein by reference.

FIELD

This invention relates to apparatuses and methods for coupling a signalto and/or from a cable which includes a plurality of twisted pairs. Inparticular, this invention relates to coupling a signal to and/or fromsuch a cable by non-contact coupling with the cable. Such signals may beused to determine interconnections, e.g. within a local area network.

BACKGROUND

Cables which include a plurality of twisted pairs, referred to as“twisted pair cables” herein, are well known. Such cables are commonlyused for telecommunications purposes, e.g. computer networking andtelephone systems. In the field of telecommunications, twisted paircables are usually provided without shielding, i.e. as unshieldedtwisted pair (UTP) cables. However, shielded twisted pair (STP) cablesare also known.

In this context, a “twisted pair” is a pair of conductors, usually aforward conductor and a return conductor of a single circuit, which havebeen twisted together. The conductors are usually twisted together forthe purposes of cancelling out electromagnetic interference fromexternal sources and to minimise cross-talk between neighbouring twistedpairs within a cable comprising a plurality of twisted pairs. In thisway, each twisted pair provides a reliable respective communicationchannel for a signal, usually a differential voltage signal, to beconveyed within the twisted pair. Common forms of unshielded twistedpair cables are category 5 and category 6 unshielded twisted pairs whichinclude eight conductors twisted together in pairs to form four twistedpairs.

The design and construction of twisted pair cables is carefullycontrolled by manufacturers to reduce noise due to electromagneticinterference and to reduce cross-talk between the twisted pairs withinthe cables. To this end, each twisted pair in a twisted pair cablenormally has a different twist rate (i.e. number of twists per unitlength along the cable) from that of the other twisted pairs in thecable. It is also usual for the twisted pairs to be twisted around eachother within the cable. Fillets or spacers may be used to separatephysically the twisted pairs.

Local area networks (LANs) are also well known. Local area networks aretypically used to enable equipment such as computers, telephones,printers and the like to communicate with each other and with remotelocations via an external service provider. Local area networkstypically utilise twisted pair network cables, usually in the form ofunshielded twisted pair cables.

The network cables in a local area network are typically connected todedicated service ports throughout one or more buildings. The networkcables from the dedicated service ports can extend through the walls,floor and/or ceilings of the building to a communications hub, typicallya communications room containing a number of network cabinets. Thenetwork cables from wall and floor sockets within the building and froman external service provider are also usually terminated within thecommunications room.

A “patch system” may be used to interconnect various network lines ofthe local area network within the network cabinets. In a patch system,all of the network lines can be terminated within the network cabinetsin an organized manner. The terminations of the network lines areprovided by the structure of the network cabinets, which are typicallyorganised in a rack system. The racks contain “patch panels”, whichthemselves utilise sets of network ports, typically RJ45-type connectorports, at which the network lines terminate.

Each of the network ports in each patch panel is hard wired to one ofthe local area network's network lines. Accordingly, each network lineis terminated on a patch panel in an organized manner. In small patchsystems, all network lines may terminate on the patch panels of the samerack. In larger patch systems, multiple racks are used, whereindifferent network lines terminate on different racks.

The interconnections between the various network lines are made using“patch cables”, which are typically unshielded twisted pair cablesincluding four twisted pairs. Each end of a patch cable is terminated bya connector, such as an RJ-45 type connector for inserting into an RJ-45type connector port as described above. One end of the patch cable isconnected to the network port of a first network line and the oppositeend of the patch cable is connected to the network port of a secondnetwork line. By selectively connecting the various network lines usingthe patch cables, a desired combination of network interconnections canbe achieved.

FIG. 1 shows a typical patch system organised into a server row 82, across-connect row 83 and a network row 84, which include patch panels 80a, 80 b, 80 c, 80 d. Patch cables 10 a, 10 b, 10 c, 10 d are used tointerconnect two network lines through the patch system.

In many businesses, employees are assigned their own computer networkaccess number so that the employee can interface with the companies ITinfrastructure. When an employee changes office locations, it is notdesirable to provide that employee with newly addressed network port.Rather, to preserve consistency in communications, it is preferred thatthe exchanges of the network ports in the employee's old office betransferred to the telecommunications ports in the employee's newlocation. This type of move is relatively frequent. Similarly, when newemployees arrive and existing employees depart, it is usually necessaryfor the patch cables in the network cabinet(s) to be rearranged so thateach employee's exchanges can be received in the correct location.

As the location of employees change, the patch cables in a typicalcabinet are often manually entered in a computer based log. This isburdensome. Further, technicians often neglect to update the log eachand every time a change is made. Accordingly, the log is often less than100% accurate and a technician has no way of reading where each of thepatch cables begins and ends. Accordingly, each time a technician needsto change a patch cable, that technician manually traces that patchcable between an internal line and an external line. To perform a manualtrace, the technician locates one end of a patch cable. The technicianthen manually follows the patch cable until he/she finds the oppositeend of that patch cable. Once the two ends of the patch cable arelocated, the patch cable can be positively identified.

It takes a significant amount of time for a technician to manually tracea particular patch cable, especially in large patch systems.Furthermore, manual tracing is not completely accurate and a technicianmay accidently go from one patch cable to another during a manual trace.Such errors result in misconnected patch cables which must be lateridentified and corrected.

Attempts have been made in the prior art to provide an apparatus whichcan automatically trace the common ends of each patch cable within localarea networks, thereby reducing the labour and inaccuracy of manualtracing procedures.

For example, U.S. Pat. No. 5,483,467 describes a patching panel scannerfor automatically providing an indication of the connection pattern ofthe data ports within a local area network, so as to avoid the manualtask of identifying and collecting cable connection information. In oneembodiment, which is intended for use with shielded twisted pair cables,the scanner uses inductive couplers which are associated with the dataports. The inductive coupler is disclosed as being operative to impose asignal on the shielding of shielded network cables in order to providean indication of the connection pattern produced by connection of thecables to a plurality of ports.

In another embodiment of U.S. Pat. No. 5,483,467, the scanner is coupledto each data port by “dry contact” with a dedicated conductor in a patchcable. This is difficult to implement in practice, because most networkcables have to meet a particular pre-determined standard in theindustry, such as RJ45, in which there is no free conductor which couldbe used for determining interconnectivity.

U.S. Pat. No. 6,222,908 discloses a patch cable identification andtracing system in which the connectors of each patch cable contain aunique identifier which can be identified by a sensor in the connectorports of a telecommunications closet. By reading the unique identifieron the connectors of each patch cable, the system can keep track ofwhich patch chords are being added to and removed from the system.Although this system avoids the use of dedicated conductors in the patchcable, it is difficult to implement because it requires use ofnon-standard patch cables, i.e. patch cables with connectors containingunique identifiers.

International Patent Application Publication Number WO00/60475 disclosesa system for monitoring connection patterns of data ports. This systemuses a dedicated conductor which is attached to the external surface ofa network cable in order to monitor the connection pattern of dataports. Although this allows the system to be used with standard networkcables, it still requires the attaching of dedicated conductors to theexternal surfaces of network cables and adapter jackets which are placedover the standard network cable.

U.S. Pat. No. 6,285,293 discloses another system and method foraddressing and tracing patch cables in a dedicated telecommunicationssystem. The system includes a plurality of tracing interface modulesthat attach to patch panels in a telecommunications closet. On the patchpanels, are located a plurality of connector ports that receive theterminated ends of patch cables. The tracing interface modules mount tothe patch panels and have a sensor to each connector port which detectswhenever a patch cable is connected to the connector port. A computercontroller is connected to the sensors and monitors and logs all changesto the patch cable interconnections in an automated fashion. However,this system cannot be retrofitted to an existing network and relies onthe operator to work in a particular order if the patch cableconnections are to be accurately monitored.

International Patent Application Publication Number WO2005/109015, whichrelates to the field of cable state testing, discloses a method ofdetermining the state of a cable comprising at least one electricalconductor and applying a generated test signal to at least one conductorof the cable by a non-electrical coupling transmitter. The reflectedsignal is then picked up and compared with expected state signal valuesfor the cable, so that the state of the cable can be determined. Theinventors have found that signals coupled to a twisted pair cable by themethods described in WO2005/109015 have a tendency to leak out from thetwisted pair cable, especially when other twisted pair cables arenearby.

SUMMARY

The present invention has been devised in light of the aboveconsiderations.

The present invention generally relates to a discovery by the inventorsthat a twisted pair cable, e.g. an unshielded twisted pair (UTP) cable,provides communication channels which are additional to the respectivecommunication channel provided within each twisted pair in the cable. Inparticular, it has been found that additional communication channelsexist between each combination of two twisted pairs within a twistedpair cable, due to coupling between the twisted pairs. This issurprising because, as explained above, twisted pair cables are designedso as to minimise cross-talk, i.e. to minimise the coupling, between thetwisted pairs. Each combination of two twisted pairs within a twistedpair cable may be termed a “pair-to-pair” combination. Therefore, theadditional communication channels may be termed “pair-to-pair” channels.

It has been found that a signal which propagates along a twisted paircable between two of the twisted pairs can propagate reliably and overuseful distances, without significantly altering the transmission ofsignals within the individual twisted pairs. In other words, such asignal can propagate in addition to the differential voltage signalswhich typically propagate within each twisted pair when the twisted paircable is in use. It is presently considered that it is the orderedgeometry typically found in twisted pair cables which allows signals topropagate usefully between two twisted pairs.

Accordingly, in general, the invention provides apparatuses and methodsfor coupling a signal to and from a twisted pair cable by non-contactcoupling with twisted pairs in the twisted pair cable, such that thesignal propagates along the cable between at least two of the twistedpairs.

When it is described that the signal propagates along a twisted paircable “between two of the twisted pairs”, it is meant that the signalpropagates along the cable due to a coupling between the two twistedpairs, the signal being a difference in state between the two twistedpairs.

For example, the signal may be a voltage signal, i.e. a difference involtage between the two twisted pairs, which propagates along the cabledue to a capacitive coupling between the two twisted pairs. As anotherexample, the signal may propagate along the cable due to an inductivecoupling between the two twisted pairs.

Accordingly, a signal which propagates between two twisted pairs isdistinguished from a so-called “common mode” signal. As discussed inmore detail below, it is also possible to convey a common mode voltagesignal along a twisted pair cable, the common mode voltage signal beinga difference in voltage between all the conductors in the twisted paircable and a ground voltage. However, in such a common mode voltagesignal, there is on average no difference in voltage between any two ofthe twisted pairs in the twisted pair cable, and therefore the commonmode voltage signal should not be seen as a voltage signal whichpropagates along a cable between two of the twisted pairs.

By “non-contact” coupling, it is intended to mean a coupling which doesnot involve direct electrical contact, i.e. does not involve ohmiccontact.

The inventors have found that a voltage signal which propagates along atwisted pair cable between twisted pairs can be non-contact coupled toand/or from the cable by a pair of electrodes, i.e. by a first electrodeand a second electrode.

To couple a voltage signal to the cable using a pair of electrodes, avoltage signal, e.g. from a voltage signal generator, may be coupled tothe electrodes so that the electrodes produce an electric fieldtherebetween. The electric field thus produced may cause a voltage to bedeveloped between two of the twisted pairs adjacent the electrodes,thereby coupling the voltage signal to the cable so that the voltagesignal propagates between at least two of the twisted pairs.

A voltage signal which propagates between at least two twisted pairs ina twisted pair cable may have an electric field associated with it. Toreceive the voltage signal, this electric field may cause a voltage tobe developed between the electrodes, thereby coupling the voltage signalfrom the cable to the electrodes. The received voltage signal maysubsequently be coupled to a voltage signal processor, for example.

A first aspect of the invention relates to a discovery by the inventorsthat the quality of a voltage signal which propagates between twistedpairs and is coupled to and/or from a twisted pair cable by a pair ofelectrodes can be improved by electrically isolating the electrodes froma voltage signal generator (when the electrodes are used to couple avoltage signal generated by the voltage signal generator to the cable)and/or by electrically isolating the electrodes from a voltage signalprocessor (when the voltage signal processor is used to process avoltage signal received from the cable by the electrodes). Inparticular, it has been found that coupling a voltage signal to atwisted pair cable using electrically isolated electrodes can help toreduce leakage of the voltage signal from the cable.

The inventors do not wish to be bound by theory, but it is thought thatthe improvement in signal quality may derive from a capacitive couplingbetween the electrically isolated electrodes and the twisted pairs suchthat the average voltage of the electrodes corresponds to the averagevoltage of the conductors which form the twisted pairs. It is alsothought that the same coupling may cause the average voltage of theelectrodes to track the average voltage of the conductors which form thetwisted pairs over time. In other words, it is thought that theimprovement may derive from the voltages of the electrodes being“balanced” with respect to the voltages of the conductors which form thetwisted pairs.

It is also thought that the same coupling may also lead to the voltagesignal which propagates along the cable between the twisted pairs havingan average voltage which corresponds to, and which tracks over time, theaverage voltage of the conductors which form the twisted pairs, i.e. sothat the average voltage of the voltage signal which propagates alongthe cable is “balanced” with respect to the voltages in the conductorswhich form the twisted pairs.

Accordingly, the first aspect of the invention provides for example acoupling unit according to claim 1.

The electrical isolation means may be any suitable means forelectrically isolating the electrodes of the coupling unit from thevoltage signal generator and/or the voltage signal processor, whilststill allowing the voltage signal to be coupled between the electrodesand the voltage signal generator and/or the voltage signal processor.The electrical isolation means may therefore include a transformer.Preferably, the electrical isolation means includes a balun.

The voltage signal coupling means may be any means suitable for couplinga voltage signal between the electrodes and a voltage signal generatorand/or voltage signal processor. The voltage signal coupling means maytherefore include one or more conductors.

Preferably, the coupling unit includes shielding for shielding theelectrodes from an external electromagnetic field. In this way, theexternal electromagnetic field can be inhibited from interfering withthe coupling between the electrically isolated electrodes and the cable.The presence of shielding has been found to improve the quality of avoltage signal which is coupled to and/or from a twisted pair cable bythe electrodes, especially in environments where there are significantexternal electromagnetic fields. The shielding may include anelectrostatic screen.

It has been found to be preferable to couple a signal to the cable bycoupling a differential voltage signal to the electrodes (a voltagesignal including two complimentary voltage signals), rather than asingle-ended voltage signal (a voltage signal including a single voltagesignal with respect to a fixed voltage, e.g. ground).

Accordingly, the voltage signal coupling means may include a convertingmeans for converting a single-ended voltage signal from a voltage signalgenerator into a differential voltage signal to be coupled to theelectrodes. In other embodiments, the voltage signal generator may beable to generate a differential voltage signal.

The converting means may additionally or alternatively be for convertinga differential voltage signal from the electrodes into a single-endedvoltage signal to be coupled to a voltage signal processor. In otherembodiments, the voltage signal processor may be able to process adifferential voltage signal.

The electrical isolating means may electrically isolate the electrodesfrom the converting means. The converting means may include a choke.

Preferably, the coupling unit has housing in which the electrodes, andoptionally any one or more of the voltage signal coupling means, theelectrical isolating means, the shielding and the converting means, ishoused.

The first and second electrodes may be spaced apart by the housing so asto allow the cable to be received therebetween. Preferably, the firstand second electrodes are spaced apart by the housing so as to belocated on directly opposite sides of the cable when the cable isreceived therebetween. In this way, the electric field produced betweenthe first and second electrodes of the transmitter can couple moreeasily with twisted pairs located on opposite sides of the cable.

The housing may be arranged to be clipped on to the cable. Preferably,the electrodes are located in the housing so as to contact an outersurface of the cable if the housing is clipped onto the cable. In thisway, the electrodes can be located in close proximity with the cable,simply by clipping a coupling unit onto the cable. The coupling unit(s)may be arranged to clip onto the cable by any suitable means, e.g. achannel in the coupling unit or retention lugs.

Preferably, each electrode includes a respective contact surface forcontacting an outer surface of the cable. Preferably, each contactsurface substantially conforms in shape to an outer surface of thecable. For example, in the case of a cable with a round, e.g. generallycircular, cross-section, the or each contact surface may have acurvature which substantially conforms to an outer surface of the cable.Therefore each electrode may be cylindrical or partially cylindrical incross section. In this way, the distance between the electrodes and thetwisted pairs in the cable can be minimised so as to improve thecoupling between the electrodes and the twisted pairs.

The coupling unit may include an amplifier, e.g. for amplifying avoltage signal to be coupled to the cable or for amplifying a voltagesignal which has been coupled from the cable. The amplifier may behoused by the housing.

The coupling unit may include a third electrode and a fourth electrode,which are preferably spaced apart from the first and second electrodes,and are preferably arranged to produce an electric field therebetween tocouple the voltage signal to the cable by non-contact coupling with thetwisted pairs so that the voltage signal propagates between at least twoof the twisted pairs. Additionally or alternatively, the third andfourth electrodes may be arranged to receive a voltage signal which haspropagated along the cable between at least two of the twisted pairs bynon-contact coupling with at least two of the twisted pairs betweenwhich the voltage signal has propagated. The inventors have found thatthis can improve the strength of the signal coupled to and/or coupledfrom the cable. The third and fourth electrodes may have any of thefeatures associated with the first and second electrodes of thetransmitter and described herein.

Although the first and second electrodes may in principle be arranged tocouple a voltage signal to and from the cable, the first and secondelectrodes may be arranged only to couple a voltage signal to the cable,in which case: the first electrode and the second electrode are arrangedto produce an electric field therebetween to couple a voltage signal tothe cable by non-contact coupling with the twisted pairs so that thevoltage signal propagates along the cable between at least two of thetwisted pairs; the voltage signal coupling means is for coupling avoltage signal generated by a voltage signal generator to theelectrodes; and the electrical isolation means is arranged toelectrically isolate the electrodes of the coupling unit from thevoltage signal generator.

In addition to electrodes arranged only to couple a voltage signal tothe cable, the coupling unit may have additional electrodes arrangedonly to couple a voltage signal from the cable. Accordingly, thecoupling unit may have: a first additional electrode and a secondadditional electrode arranged to receive a voltage signal which haspropagated along the cable between at least two of the twisted pairs bynon-contact coupling with at least two of the twisted pairs betweenwhich the voltage signal has propagated; and a voltage signal couplingmeans for coupling a voltage signal received by the additionalelectrodes to a voltage signal processor; wherein the voltage signalcoupling means includes electrical isolation means arranged toelectrically isolate the additional electrodes of the coupling unit fromthe voltage signal processor. The first and second additional electrodesmay have any of the features described in connection with reference tothe first and second electrodes.

In other embodiments, the first and second electrodes may be arrangedonly to couple a voltage signal from the cable, in which case: the firstelectrode and the second electrode are arranged to receive a voltagesignal which has propagated along the cable between at least two of thetwisted pairs by non-contact coupling with at least two of the twistedpairs between which the voltage signal has propagated; the voltagesignal coupling means is for coupling a voltage signal received by theelectrodes to a voltage signal processor; and the electrical isolationmeans is arranged to electrically isolate the electrodes of the couplingunit from the voltage signal processor.

The voltage signal coupling means may couple (directly or indirectly) avoltage signal generator to the electrodes of the coupling unit. Thevoltage signal generator may be arranged to generate a voltage signal tobe coupled to the cable by the electrodes. Therefore, there may beprovided an apparatus having: a coupling unit as described herein; and avoltage signal generator; wherein the voltage signal coupling meanscouples the voltage signal generator to the electrodes of the couplingunit.

The voltage signal generator may be arranged to generate a single-endedvoltage signal, e.g. to be converted into a differential voltage signalby a converting means. In some embodiments, the voltage signal generatormay be arranged to generate a differential voltage signal.

The voltage signal generator may be arranged to generate a data signalwhich contains data, e.g. digital data. In this way, two of the couplingunits can be used to transmit data from a first location to a secondlocation along the twisted pair cable.

The data signal may contain address data to identify the coupling unit.This is particularly useful if the apparatus is to be used to identifyan interconnection between two coupling units, as described in moredetail below.

The voltage signal generator may be arranged to generate a test signalfor determining a state of the cable. For example, the test signal maybe a signal that is suitable for use in reflectometry. Reflectometrytechniques are well-known in the art and include time domainreflectometry and frequency domain reflectometry. Accordingly, the testsignal may be suitable for performing time domain reflectometry orfrequency domain reflectometry. Some reflectometry techniques aredescribed, for example, in International Patent Application PublicationNumber WO2005/109015.

It has been found that performing reflectometry with signals whichpropagate between at least two twisted pairs in a cable can produceparticularly clear results compared with performing reflectometry withsignals which propagate within individual twisted pairs in a twistedpair cable.

The voltage signal coupling means may couple (directly or indirectly) avoltage signal processor to the electrodes of the coupling unit. Thevoltage signal processor may be arranged to process a voltage signalreceived by the electrodes. Therefore, there may be provided anapparatus having: a coupling unit as described herein; and a voltagesignal processor; wherein the voltage signal coupling means couples thevoltage signal processor to the electrodes of the coupling unit.

The same electrodes could be coupled to both a voltage signal generatorand a voltage signal processor, e.g. if the same electrodes are to beused for coupling a signal to and coupling a signal from the cable.

The voltage signal processor may be arranged to receive a single-endedvoltage signal, e.g. converted from a differential voltage signal by thecoupling unit. In some embodiments, the voltage signal processor may bearranged to receive a differential voltage signal.

The voltage signal processor may include a data retrieving means forretrieving data from a data signal received by the receiver, e.g. bydecoding and/or demodulating a signal received by the receiver.

The voltage signal processor may be arranged to determine a state of acable based on a test signal received by the receiver. For example, thevoltage signal processor may be arranged to determine a state of thecable by comparing a received signal with a reference signal, as isknown in the art of reflectometry.

There may be provided an apparatus having: a cable including a pluralityof twisted pairs; a first coupling unit for coupling a voltage signal tothe cable; a second coupling unit for coupling the voltage signal fromthe cable. The first and second coupling units may have any of thefeatures described herein in connection with coupling units. The cablemay be an unshielded twisted pair cable, e.g. including eight conductorstwisted together to form four twisted pairs. Such cables are well known.

The first and second coupling units may be located at differentpositions along the cable. This allows the first and second couplingunits to be used to transmit data along the cable, e.g. if the voltagesignal is a data signal. If the voltage signal is a test signal, it maybe useful to have the first and second coupling units located at asimilar or the same position along the cable, e.g. to performreflectometry.

A second aspect of the invention generally relates to an apparatus forcoupling a signal which propagates between twisted pairs to and from atwisted pair cable. Accordingly, a second aspect of the invention mayprovide for example an apparatus for coupling a signal to and from acable according to claim 28.

Prior art apparatuses known to the inventors are not arranged to receivea signal which has propagated between twisted pairs.

For example, International Patent Application Publication NumberWO2005/109015, described above, discloses a method of applying agenerated test signal to at least one conductor of a twisted pair cableby a non-electrical coupling transmitter, picking up a reflected signaland comparing it with expected state signal values for the cable todetermine its current state. However, this patent application does notdisclose an apparatus arranged to receive a signal which has propagatedalong a twisted pair cable between at least two twisted pairs.

The signal may be a voltage signal, i.e. with the first coupling unitarranged to couple a voltage signal to the cable and the second couplingunit arranged to couple the voltage signal from the cable.

For reasons explained above, it is currently thought that an improvementin the quality of a voltage signal coupled to and/or from a twisted paircable may derive from a voltage signal which propagates along the cablebetween twisted pairs having an average voltage which corresponds to,and which may track over time, the average voltage of the conductorswhich form the twisted pairs.

Accordingly, the first coupling unit may be arranged such that thevoltage signal which propagates between at least two of the twistedpairs has an average voltage which corresponds to the average voltage inthe conductors which form the twisted pairs. The first coupling unit mayfurther be arranged such that the voltage signal which propagatesbetween at least two of the twisted pairs has an average voltage whichtracks the average voltage in the conductors which form the twistedpairs over time.

The first coupling unit may have a first electrode and a secondelectrode arranged to produce an electric field therebetween to couple avoltage signal to the cable by non-contact coupling with the twistedpairs so that the voltage signal propagates along the cable between atleast two of the twisted pairs.

The first coupling unit may be arranged such that the first and secondelectrodes have an average voltage which corresponds to the averagevoltage in the conductors which form the twisted pairs, e.g. to improvethe quality of the voltage signal which propagates between twisted pairsfor reasons discussed above. The first coupling unit may be arrangedsuch that the first and second electrodes have an average voltage whichtracks the average voltage in the conductors which form the twistedpairs over time, e.g. to improve the quality of the voltage signal whichpropagates between twisted pairs for reasons discussed above.

The first coupling unit may have or be associated with any of thefeatures described with reference to the first aspect of the invention.However, the electrical isolating means described with reference to thefirst aspect may be omitted, since it has been found that it is stillpossible to couple a voltage signal to a twisted pair cable with theelectrical isolating means omitted, even though this may lead to areduction in the quality of the voltage signal which propagates betweenthe twisted pair.

The second coupling unit may have a first electrode and a secondelectrode arranged to receive a voltage signal which has propagatedalong the cable between at least two of the twisted pairs by non-contactcoupling with at least two of the twisted pairs between which thevoltage signal has propagated.

The second coupling unit may be arranged such that the first and secondelectrodes have an average voltage which corresponds to the averagevoltage in the conductors which form the twisted pairs, e.g. to improvethe quality of the voltage signal received by the second coupling unitfor reasons discussed above. The second coupling unit may be arrangedsuch that the first and second electrodes have an average voltage whichtracks the average voltage in the conductors which form the twistedpairs over time, e.g. to improve the quality of the voltage signalreceived by the second coupling unit for reasons discussed above.

The second coupling unit may have or be associated with any of thefeatures described with reference to the first aspect of the invention.However, the electrical isolating means described with reference to thefirst aspect may be omitted, since it has been found that it is stillpossible to couple a voltage signal from a twisted pair cable with theelectrical isolating means omitted, even though the quality of thevoltage signal may be reduced.

The second aspect of the invention may provide a first coupling unit forcoupling a signal to a cable including a plurality of twisted pairshaving any of the features described above or may provide a secondcoupling unit for coupling a signal from a cable having any of thefeatures described above.

A third aspect of the invention relates to using a plurality of thecoupling units described herein to determine interconnections by twistedpair cables, e.g. in a local area network. An advantage of using thecoupling units described herein to determine interconnections by twistedpair cables is that the signals which are coupled to and from a twistedpair cable by such coupling units can propagate in addition to thedifferential voltage signals which are typically present within eachtwisted pair when the twisted pair cables are in use. In this way,interconnections can be determined using a standard twisted pair cable.

Accordingly, the third aspect of the invention may provide an apparatushaving: one or more first coupling units for coupling a signal to acable including a plurality of twisted pairs so that the signalpropagates along the cable between at least two of the twisted pairs;one or more second coupling units for coupling a signal which haspropagated between at least two twisted pairs from a cable including aplurality of twisted pairs; and an interconnection determining meansarranged to determine one or more interconnections between any one ofthe first coupling units and any one of the second coupling units by oneor more cables including a plurality of twisted pairs

The first coupling unit may have or be associated with any of thefeatures described in the first and/or second aspects of the invention,e.g. it may be arranged to couple a signal to the cable by non-contactcoupling with the twisted pairs so that the signal propagates along thecable between at least two of the twisted pairs. The second couplingunit may have or be associated with any of the features described in thefirst and/or second aspects of the invention, e.g. it may be arranged toreceive a signal which has propagated along a cable including aplurality of twisted pairs by non-contact coupling with at least two ofthe twisted pairs between which the signal has propagated.

Although both the first and the second coupling units may be arranged tonon-contact couple with a cable including a plurality of twisted pairs,it is possible for only the first coupling units or only the secondcoupling units to be arranged to non-contact couple with a cableincluding a plurality of twisted pairs. In this case, the coupling unitswhich are not arranged to non-contact couple with the twisted pairs maybe arranged to couple with the twisted pairs by direct electrical, i.e.ohmic, contact. A preferred arrangement may have the one or more firstcoupling units being arranged to couple with the twisted pairs by directelectrical contact. Where the apparatus is for use in a local areanetwork, the one or more coupling units which are arranged to couplewith the twisted pairs by direct electrical contact could be integratedwithin a known type of network apparatus, e.g. a switch unit.

The interconnection determining means is preferably arranged todetermine an interconnection between one of the first coupling units andone of the second coupling units by determining if a second couplingunit couples a signal from a cable which has been coupled to the cableby the first coupling unit. In other words, the coupling of a signalfrom the cable by a second coupling unit, which signal has been coupledto the cable by a first coupling unit, is preferably used by theinterconnection determining means to identify an interconnection betweenthe first and second coupling unit.

The interconnection determining means may be a suitably programmedprocessing means, e.g. computer, connected to the first coupling unit(s)and second coupling unit(s).

Each of the one or more first coupling units may be arranged to coupleto the cable a data signal containing address data to identify the firstcoupling unit. In this way, the signal received by the second couplingunit can be used to identify the first coupling unit which coupled thesignal to the cable. The first coupling unit may be arranged to couplethe data signal to the cable by being coupled to a voltage signalgenerator, for example.

Preferably, the apparatus is for determining interconnections betweennetwork ports in a local area network. Accordingly, each of the one ormore first coupling units and each of the one or more second couplingunits may be associated with a respective network port in a local areanetwork. Furthermore, the interconnection determining means may bearranged to determine one or more interconnections between the networkports by one or more cables including a plurality of twisted pairs. Inthis way, the apparatus can be used to determine interconnectionsbetween network ports by patch cables in a local area network, withoutrequiring the use of special patch cables or special patch panels andwithout the transmission of differential voltage signals within eachtwisted pair necessarily being interrupted.

The apparatus may include one or more signal units. Each signal unit mayinclude a plurality of the first coupling units and/or a plurality ofthe second coupling units. The signal unit may include a voltage signalgenerator and/or a voltage signal processor. The invention may provideone of the signal units on its own. The signal unit may be integratedwithin a known type of network apparatus, e.g. a network switch.

A fourth aspect of the invention provides a kit of parts for forming anapparatus as set out in any one of the first, second or third aspects ofthe invention.

A fifth aspect of the invention generally relates to methods of couplinga signal to and/or from a cable including a plurality of twisted pairsusing any apparatus disclosed herein, particularly those apparatusesdescribed in connection with the above aspects of the invention.

Accordingly, the fifth aspect of the invention may provide a method ofcoupling a voltage signal to and/or from a cable including a pluralityof twisted pairs, the method including: producing an electric field tocouple a voltage signal to the cable by non-contact coupling with thetwisted pairs so that the voltage signal propagates along the cablebetween at least two of the twisted pairs; and/or receiving a voltagesignal which has propagated along the cable between at least two of thetwisted pairs by non-contact coupling with at least two of the twistedpairs between which the voltage signal has propagated.

The fifth aspect of the invention may provide a method of coupling asignal to and from a cable including a plurality of twisted pairs, themethod including: coupling a signal to the cable by non-contact couplingwith the twisted pairs so that the signal propagates along the cablebetween at least two of the twisted pairs; and receiving the signalwhich has propagated along the cable by non-contact coupling with atleast two of the twisted pairs between which the signal has propagated.

The invention also includes any combination of the aspects and preferredfeatures described except where such a combination is clearlyimpermissible or expressly avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of our proposals are discussed below, with reference to theaccompanying drawings in which:

FIG. 1 is a representation of a patch system.

FIGS. 2 a and 2 b are representations of a twisted pair cableillustrating the difference between propagation of differential voltagesignals within the twisted pairs of the twisted pair cable and thepropagation of a common mode voltage signal in the twisted pair cable.

FIGS. 3 a and 3 b are representations of an apparatus for coupling acommon mode voltage signal to and from a twisted pair cable.

FIG. 4 is a representation illustrating the propagation of a voltagesignal along a twisted pair cable between two twisted pairs in thetwisted pair cable.

FIG. 5 a is a representation of a pair of electrodes for coupling avoltage signal which propagates between twisted pairs to a twisted paircable.

FIG. 5 b is a representation of two of the pairs of the electrodes shownin FIG. 5 a.

FIG. 6 is a representation of a coupling unit for coupling a voltagesignal which propagates between twisted pairs to and/or from a twistedpair cable.

FIG. 7 is a representation of a network apparatus for identifyinginterconnections between network ports in a local area network.

FIG. 8 is a representation of a signal unit of the network apparatusshown in FIG. 7.

FIG. 9 is a representation of a coupling unit and circuitry associatedwith the coupling unit in the signal unit of FIG. 8.

FIG. 10 is a representation of an equivalent circuit model of a shortelemental section of a twisted pair cable.

FIG. 11 is a representation of a twisted pair cable to be modelled usingthe equivalent circuit model of FIG. 10.

FIGS. 12 a-c are graphs showing the signal propagation along a twistedpair cable modelled using the equivalent circuit model of FIG. 10.

FIG. 13 is a representation of a test apparatus for coupling a voltagesignal which propagates between twisted pairs to and from a twisted paircable.

FIGS. 14, 15 and 16 are time domain reflectometry plots produced usingthe test apparatus of FIG. 13.

FIG. 17 is a representation of a matched inter-pair termination resistornetwork used to produce the reflectometry plot of FIG. 16 b.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIGS. 2 a and 2 b each show a twisted pair cable having eight conductors1 to 8 which are twisted together in pairs to form four twisted pairs1-2, 3-4, 5-6, 7-8.

In FIG. 2 a, the twisted pair cable is being used as it was originallyintended to be used, i.e. with a respective differential voltage signalpropagating within each of the twisted pairs 1-2, 3-4, 5-6, 7-8. Whenthe twisted pair cable is being used in this way, the two conductors ineach twisted pair act as the forward (+) and the return (−) path of asingle circuit, and therefore allow a differential voltage signal topropagate within the twisted pair. The differential voltage signalpropagating within each twisted pair can be considered as the differencein voltage between the conductors in the twisted pair, e.g. as V1−V2,V3−V4, V5−V6 or V7−V8, where V1 to V8 represents the voltages inconductors 1 to 8 with respect to a reference voltage.

Twisted pair cables are typically very carefully designed andconstructed such that each twisted pair has a desired characteristicimpedance, e.g. 100 Ohms, and so that the differential voltage signalwhich propagates within each twisted pair has a desired velocity ofpropagation, e.g. 0.7 times the speed of light in vacuum.

In FIG. 2 b, the twisted pair cable is being used such that a commonmode voltage signal is propagating through the twisted pair cable. Whena common mode voltage signal is propagating, all of the conductors inthe twisted pairs act as the forward (+) path of the circuit, with alocal ground (GND) acting as the return (−) path for the circuit. Thecommon mode voltage signal can be considered as the difference involtage between all the conductors and the local ground, e.g. as(V1+V2+V3+V4+V5+V6+V7+V8)/8, where V1 to V8 represents the voltages inconductors 1 to 8 with respect to the local ground.

FIG. 3 a shows an apparatus for coupling a common mode voltage signal toand from a twisted pair cable 10. The twisted pair cable 10 shown inFIG. 3 a is an unshielded twisted pair cable including four twistedpairs. The apparatus includes a first coupling unit 20 for coupling acommon mode voltage signal to the twisted pair cable 10 at a firstlocation on the twisted pair cable 10, and a second coupling unit 40 forcoupling a common mode voltage signal from the twisted pair cable 10 ata second location on the twisted pair cable 10. The first coupling unit20 is connected to a voltage source 30 for generating a single-endedvoltage signal to be coupled to the twisted pair cable 10 by the firstcoupling unit 20. The second coupling unit 40 is connected to anamplifier 50 for amplifying a voltage signal coupled from the twistedpair cable 10 by the second coupling unit 40.

Also shown in FIG. 3 a is a local ground TGND for the first couplingunit 20 and a local ground RGND for the second coupling unit 40. Thelocal grounds TGND and RGND for the first coupling unit 20 and thesecond coupling unit 40 are not isolated from each other and thereforeshare a common ground 60, which is illustrated in FIG. 3 a by a dashedline.

The first coupling unit 20 and the second coupling unit 40 each have arespective (single) antenna which encircles the twisted pair cable 10.Because the first coupling unit 20 has a single antenna, in use, itproduces the same voltage in each of the twisted pairs in the twistedpair cable 10. Consequently, the first coupling unit 20 couples a commonmode voltage signal to the twisted pair cable 10, rather than a voltagesignal which propagates between the twisted pairs in the twisted paircable 10.

When a common mode voltage signal is coupled to the twisted pair cable10 by the first coupling unit 20, the return path for the common modevoltage signal is unclear. Consequently, the common mode voltage signalmay flow out of the twisted pair cable 10 such that the common ground 60acts as a return path for the common mode voltage signal. Interference62 from other ground currents in the common ground 60 may interfere withthe common mode voltage signal.

Consequently, the strength of a common mode voltage signal received bythe second coupling unit 40 is not a definitive indicator that the firstcoupling unit 20 and second coupling unit 40 are interconnected by atwisted pair cable.

In any case, the coupling of a common mode voltage signal to the twistedpair cable 10 raises issues associated with electromagneticcompatibility, since common mode voltage signals are normally avoidedand/or seen as a potential source of interference in twisted paircables.

FIG. 3 b shows the same apparatus as FIG. 3 a, which has been modifiedto demonstrate a further problem associated with coupling a common modevoltage signal to and from the twisted pair cable 10. In FIG. 3 b, thefirst coupling unit 20 is coupled to a first twisted pair cable 10 awhich is terminated by a port A. The second coupling unit 40 is coupledto a second twisted pair cable 10 b which is terminated by a port B. Afurther twisted pair cable (not shown) can be coupled between the portsA and B to allow common mode voltage signals to propagate along thecable between the first coupling unit 20 and the second coupling unit40. FIG. 3 b also shows nearby twisted pair cables 12, which arecapacitively coupled to the first and second twisted pair cables 10 aand 10 b. This coupling is represented in the FIG. 3 b by a pair ofcapacitors 14 a and 14 b. In FIG. 3 b, the common ground 60 issymbolically represented by an inductor.

As can be seen from FIG. 3 b, if a common mode voltage signal propagatesin the first twisted pair cable 10 a, then the signal is not constrainedto stay within the first twisted pair cable 10 a because of thecapacitive coupling with the nearby twisted pair cables 12. Therefore,the nearby twisted pair cables 12 provide an unwanted pathway for thecommon mode voltage signal to be transmitted from the first couplingunit 20 to the second coupling unit 40.

As a result, a common mode voltage signal from the first coupling unit20 may be conveyed to the second coupling unit 40, even if a twistedpair cable is not connected between ports A and B. Consequently, thestrength of a common mode voltage signal received by the second couplingunit 40 is not a definitive indicator that the ports A and B areinterconnected by a twisted pair cable.

FIG. 4 illustrates the propagation of a voltage signal 111 along atwisted pair cable 110 between two twisted pairs in the twisted paircable 110. The twisted pair cable 110 shown in FIG. 4 is an unshieldedtwisted pair cable, e.g. a category 5 or category 6 unshielded twistedpair cable, including eight conductors 1 to 8 which are twisted togetherin pairs to form four twisted pairs 1-2, 3-4, 5-6, 7-8.

As illustrated in FIG. 4, each twisted pair has a twist rate which isdifferent from that of the other twisted pairs. The twisted pairs in thecable 110 are additionally twisted around each other, but for clarity,this is not illustrated in FIG. 4 (this additional twisting is shown inFIG. 5 b).

As shown in FIG. 4, the voltage signal 111 propagates along the twistedpair cable 110 between the twisted pairs 1-2 and 3-4. The voltage signal111 can be considered as the difference in voltage between the twistedpairs 1-2 and 3-4, e.g. as ((V1+V2)/2)−((V3+V4)/2), where V1 to V8represents the voltages in conductors 1 to 8 with respect to a referencevoltage.

Compared with the situation when a common mode voltage signal is coupledto a twisted pair cable, the voltage signal 111 has a more clearlydefined channel through which it propagates, i.e. the pair-to-pairchannel between the twisted pairs 1-2 and 3-4. Consequently, it isthought that the amount of leakage of the voltage signal 111 from thetwisted pair cable 110, e.g. through coupling within any nearby cables,is reduced compared with a common mode voltage signal.

Because the twisted pair cable 110 includes four twisted pairs 1-2, 3-4,5-6, 7-8, there are, in theory, six distinct pair-to-pair combinationsof twisted pairs which could act as distinct communication channels fora voltage signal to propagate between two pairs. These combinations oftwisted pairs are as follows:

Pairs 1-2 and 3-4;

Pairs 1-2 and 5-6;

Pairs 1-2 and 7-8;

Pairs 3-4 and 5-6;

Pairs 3-4 and 7-8; and

Pairs 5-6 and 7-8.

Each of these pair-to-pair combinations may have its own value ofvelocity of propagation and characteristic impedance, these values beingdetermined by the geometry of the cable and the dielectric properties ofany insulation used in the cable.

In practice, it has been found that significant cross-talk between thepair-to-pair channels within a twisted pair cable can occur such that,for a particular twisted pair cable, it may be desirable to couple onlyone voltage signal which propagates between the twisted pairs to thetwisted pair cable 110 at any one time, this voltage signal being acombination, i.e. superposition, of voltage signals which propagatebetween all pair-to-pair combinations. However, the possibility ofsimultaneously conveying a plurality of voltage signals betweendifferent pair-to-pair combinations within a twisted pair cable isenvisaged as a possibility.

FIG. 5 a shows a pair of electrodes 120 a and 120 b for coupling avoltage signal which propagates between twisted pairs to and from atwisted pair cable.

As shown in FIG. 5 a, the first electrode 120 a is provided in the formof a first plate and the second electrode 120 b is provided 120 b in theform of a second plate. The electrodes 120 a and 120 b together form acapacitor. In this example, the plates forming the first and secondelectrodes 120 a, 120 b are approximately 20 mm long and 8 mm wide. Theplates may be made of any suitable material e.g. copper foil.

The first and second electrodes 120 a and 120 b are spaced apart toallow the cable 110 to be received therebetween, such that theelectrodes 120 a and 120 b are located on directly opposite sides of thetwisted pair cable 110. Each of the plates forming the electrodes 120 aand 120 b has an inwardly curved (i.e. concave) contact surface forcontacting the convex outer surface 112, in this case an insulatingsheath, of the twisted pair cable 110. The curvature of the contactsurfaces of the plates conform to the curvature of the outer surface ofthe twisted pair cable 110 so that the electrodes 120 a and 120 b can beheld in contact with the outer surface 112.

To couple a signal into the cable 110 by non-contact coupling with thetwisted pairs, a voltage signal, e.g. from a voltage signal generator,may be coupled to the electrodes 120 a, 120 b so that a correspondingelectric field is produced between the electrodes 120 a, 120 b. Becausethe electric field between the first and second electrodes 120 a, 120 bis different at twisted pairs 1-2 and 5-6, a voltage is developedbetween twisted pairs 1-2 and 5-6 which corresponds to the voltagesignal coupled to the electrodes 120 a, 120 b. In this way, the voltagesignal can be coupled to the cable 110 such that it propagates betweenat least twisted pairs 1-2 and 5-6.

The electrodes 120 a, 120 b may additionally or alternatively be used toreceive a signal which has propagated between at least two of thetwisted pairs in the twisted pair cable 110 by non-contact coupling withat least two of the twisted pairs between which the signal haspropagated, as shall now be described with reference to a voltage signalthat is propagating between the twisted pairs 1-2 and 5-6.

The voltage signal propagating between twisted pairs 1-2 and 5-6 of thecable 110 will have an electric field 121 between the twisted pairs 1-2and 5-6 associated therewith. The electric field 121 may cause a voltageto be developed between the first and second electrodes 120 a, 120 bwhich corresponds to the voltage signal between the twisted pairs 1-2and 5-6. In this way, the voltage signal can be received by theelectrodes 120 a, 120 b.

FIG. 5 b shows two of the pairs of the electrodes 120 a, 120 b and 140a, 140 b shown in FIG. 5 a. Electrodes 120 a, 120 b may be used as theelectrodes of a first coupling unit for coupling a voltage signal to thecable 110 to propagate along the twisted pair cable 110 between at leasttwo of the twisted pairs in the twisted pair cable 110. Electrodes 140a, 140 b may be used as the electrodes of a second coupling unit forcoupling the voltage signal from the twisted pair cable 110 after it haspropagated along the twisted pair cable 110.

FIG. 5 b also shows the twisted pair cable 110 of FIGS. 4 and 5 a inmore detail. As shown in FIG. 5 b, not only is each twisted pair 1-2,3-4, 5-6, 7-8 twisted at a twist rate which is different to that of theother twisted pairs, but all of the twisted pairs are additionallytwisted around each other. This is typical in an unshielded twisted paircable.

Because all the twisted pairs 1-2, 3-4, 5-6, 7-8 of the twisted paircable 110 are twisted around each other, the electrodes 140 a and 140 bof the second coupling unit are not necessarily aligned to be adjacentto the same twisted pairs as the electrodes 120 a and 120 b of the firstcoupling unit. Consequently, the strength of the signal receivable bythe electrodes 140 a and 140 b of the second coupling unit variesbetween maxima and minima according to their longitudinal position alongthe twisted pair cable 110. Varying the circumferential position of theelectrodes 140 a, 140 b has a similar effect.

In practice, the inventors have found that a signal of adequate strengthcan often be received irrespective of the longitudinal/circumferentialposition of the electrodes 140 a and 140 b of the second coupling unit.However, the above-described maxima and minima effect may lead to “null”locations on the twisted pair cable at which the electrodes 140 a and140 b of the second coupling unit cannot receive a signal. Thus, it maybe necessary to adjust the longitudinal/circumferential position of theelectrodes 140 a and 140 b of the second coupling unit in order toreceive a signal having a desired strength.

An alternative solution, which avoids the need to adjust thelongitudinal/circumferential position of the electrodes 140 a and 140 bof the second coupling unit, is to have two pairs of electrodes, i.e.four electrodes in total, for coupling a voltage signal to and/or fromthe twisted pair cable 110 (not shown). For example, if there are twopairs of electrodes for coupling the voltage signal from the twistedpair cable, an appropriate longitudinal separation between the two pairsof electrodes could be chosen to ensure that if the first pair ofelectrodes was in a “null” position, then the second pair of electrodeswould be near a maximum. A detector and/or a switch could be used toallow the pair of electrodes receiving the largest signal to beselected, e.g. by a voltage signal processor.

FIG. 6 shows a coupling unit 120 for coupling a voltage signal whichpropagates between at least two twisted pairs to and/or from the twistedpair cable 110 of FIGS. 4 and 5.

The coupling unit 120 includes a first electrode 120 a and a secondelectrode 120 b, which may be as described with reference to FIGS. 5 aand 5 b. The coupling unit 120 preferably includes a voltage signalcoupling means which may include a first terminal 125 a, a secondterminal 125 b, an electrical isolating means 122 in the form of abalun, and a converting means 124 in the form of a choke. The couplingunit 120 preferably includes shielding 129 in the form of anelectrostatic screen which encloses the electrodes 120 a, 120 b, theelectrical isolating means 122 and the converting means 124, and ispreferably connected to the local ground GND. A suitable balun for theelectrical isolating means 122 may be Mini-Circuits® type MCL506T2-T1. Asuitable choke for the converting means 124 may be Mini-Circuits® typeMCL750T1-1.

To couple a voltage signal to the twisted pair cable 110, the firstterminal 125 a may be connected to a voltage signal generator (notshown). The second terminal 125 b may be connected to a local ground GNDfor the signal generator.

The voltage signal generated by the voltage signal generator may be asingle-ended voltage signal which is converted into a differentialvoltage signal by the converting means 124, i.e. the choke, as is knownto those skilled in the art. For example, if the signal generatorproduced a sinusoidal voltage expressed (in complex phasor notation) asV.exp(jωt); then the voltages outputted by the converting means 124 maybe expressed as V.exp(jωt)/2 and −V.exp(jωt). The differential voltagesignal from the converting means 124 is then coupled to the electrodes120 a, 120 b via the electrical isolating means 122, which electricallyisolates the electrodes from the voltage signal generator.

As previously explained, the inventors have found that the quality ofthe voltage signal coupled to the twisted pair cable 110 by the couplingunit 120 can be improved significantly as a consequence of electricallyisolating the electrodes 120 a, 120 b. In particular, it has been foundthat coupling the voltage signal to the twisted pair cable 110 byelectrically isolated electrodes 120 a, 120 b can significantly reducethe amount of leakage of the voltage signal from the twisted pair cable110, e.g. through neighbouring twisted pair cables.

To couple a voltage signal from the twisted pair cable 110, the firstterminal 125 a may be connected to a signal processor (not shown) forprocessing a voltage signal received by the electrodes 120 a, 120 b ofthe coupling unit 120. The second terminal 125 b may be connected to alocal ground GND for the signal processor.

The electrodes 120 a, 120 b of the coupling unit 120 may receive avoltage signal from the twisted pair cable 110 as previously described.The received voltage signal can then be coupled to the voltage signalprocessor via the balun 122 and the choke 124. The voltage signalreceived by the electrodes 120 a, 120 b may be a differential voltagesignal which may be converted to a single-ended voltage by theconverting means 124, i.e. the choke, as is known to those skilled inthe art.

As previously explained, the inventors have found that the quality ofthe voltage signal coupled from the twisted pair cable 110 by thecoupling unit 120 can be improved significantly as a consequence ofelectrically isolating the electrodes 120 a, 120 b.

FIG. 7 shows a network apparatus for identifying interconnectionsbetween network ports in a local area network.

The local area network shown in FIG. 7 includes a plurality of firstpatch panels 280 a which are connected to server lines 282, and aplurality of second patch panels 280 b which are connected to networklines 284. Each patch panel 280 a, 280 b has a plurality of networkports. The network ports of the first patch panels 280 a and the networkports of the second patch panels 280 b are interconnected by a pluralityof patch cables 210. However, for clarity, only one of the patch cables210 is illustrated in FIG. 7. In this example, each patch cable 210 isan unshielded twisted pair cable, e.g. a category 5 or category 6unshielded twisted pair cable, including four twisted pairs.

The network apparatus shown in FIG. 7 includes a plurality of signalunits 290 connected to a control unit 292 which is provided in the formof a suitably programmed computer. Each signal unit 290 is associatedwith a respective one of the patch panels 280 a, 280 b in the local areanetwork. Each signal unit 290 may be held in place next to the patchpanel 280 with which it is associated by any suitable means, e.g. cableties, preferably so as to avoid applying excessive strain to the cablesof the server lines 282 and network lines 284.

FIG. 8 shows a signal unit 290 of FIG. 7 in more detail. As shown inFIG. 8, the signal unit 290 includes a plurality of coupling units 220for coupling a voltage signal which propagates between twisted pairs toand from a patch cable. Each coupling unit 220 is associated with arespective network port in the patch panel with which its signal unit290 is associated.

FIG. 9 shows a coupling unit 220 and the circuitry associated with thecoupling unit 220 in the signal unit 290 of FIG. 8.

As shown in FIG. 9, the coupling unit 220 includes a first pair ofelectrodes 220 a, 220 b for coupling a voltage signal to the patch cable210, a second pair of electrodes 240 a, 240 b for coupling a voltagesignal from the patch cable 210, and shielding 229. The shielding 229separates the first pair of electrodes 220 a, 220 b from the second pairof electrodes 240 a, 240 b and reduces direct coupling between the firstpair of electrodes 220 a, 220 b and the second pair of electrodes 240 a,240 b. The shielding also shields the electrodes from externalelectromagnetic fields, e.g. from nearby patch cables 210 and nearbycoupling units 220.

The coupling unit 220 preferably has a housing arranged to be clipped onto one of the patch cables 210 (e.g. by way of a suitable channel in thecoupling unit or suitable retention lugs) such that the first pair ofelectrodes 220 a, 220 b and the second pair of electrodes 240 a, 240 bcontact an outer surface of the patch cable 210. The first pair ofelectrodes 220 a, 220 b are preferably located on directly oppositesides of the cable 210. Likewise the second pair of electrodes 240 a,240 b are preferably located on directly opposite sides of the cable210.

For coupling a voltage signal to a patch cable 210 via the first pair ofelectrodes 220 a, 220 b, the coupling unit 220 further has an electricalisolating means 222 in the form of a balun, a converting means 224 inthe form of a choke, and an amplifier 226. The first pair of electrodes220 a, 220 b, the electrical isolating means 222 and the convertingmeans 224 may be similar to those described with reference to FIGS. 5and 6. FIG. 9 shows the electrical isolating means 222, the convertingmeans 224 and the amplifier 226 of the coupling unit 220 as not beinghoused in the housing of the coupling unit 220. However, it is equallypossible for these components to be housed in the housing of thecoupling unit 220, e.g. as shown in FIG. 6.

The circuitry in a signal unit 290 for coupling a voltage signal from apatch cable 210 via the second pair of electrodes 240 a, 240 b includesan electrical isolating means 242 in the form of a balun, a convertingmeans 244 in the form of a choke, and an amplifier 246. The second pairof electrodes 240 a, 240 b, the electrical isolating means 222 and theconverting means 224 may be similar to those described with reference toFIGS. 5 and 6. FIG. 9 shows the electrical isolating means 242, theconverting means 244 and the amplifier 246 of the coupling unit 220 asnot being housed in the housing of the coupling unit 220. However, it isequally possible for these components to be housed in the housing of thecoupling unit 220, e.g. as shown in FIG. 6.

The circuitry in the signal unit 290 preferably includes one or more ofa processor 270, a field programmable gate array 272, a direct signalsynthesizer 274, a multiplexer 275, a multiplier 276, a low pass filterand amplifier 278, and an analogue to digital converter 279 all of whichare preferably connected as shown in FIG. 9. The processor 270 ispreferably connected to, and controlled by, the control unit 292 of FIG.7. The multiplier 276, low pass filter and amplifier 278, analogue todigital converter 279, field programmable gate array 272 and processor270 of the signal unit 290 may be shared by the all coupling units 220in the signal unit 290.

The processor 270, field programmable gate array 272, direct signalsynthesizer 274 and multiplexer 275 together form a voltage signalgenerator arranged to generate a voltage signal to be coupled to a patchcable 210 by the electrodes 220 a, 220 b of the coupling unit 220. Theprocessor 270, multiplexer 275, multiplier 276, low pass filter andamplifier 278, and analogue to digital converter 279 together form avoltage signal processor arranged to process a voltage signal coupledfrom the patch cable 210 by the electrodes 240 a, 240 b of the couplingunit 220.

The voltage signal generator may be arranged to generate a single-endedvoltage signal to be supplied to the coupling unit 220. Preferably, thevoltage signal generator is arranged to selectably generate either adata signal or to generate a test signal for determining the state ofthe cable 210.

When the voltage signal generator generates a data signal, the directsignal synthesizer 274 is preferably controlled by the fieldprogrammable gate array 272 and processor 270 to generate a data signalcontaining address data to identify the coupling unit 220. There aremany approaches for modulating and coding the data signal betweencoupling units, which are well known to those skilled in the art. Insome embodiments, the data signal is generated using frequency shiftkeying modulation, e.g. by generating a carrier signal having a firstfrequency to represent “0” and a second frequency to represent “1”. Toimprove the discrimination of the signal from interference or spuriouscoupling from other cables, the data signal could be transmitted overseveral frequencies using spread spectrum techniques.

When the voltage signal generator is in use to generate a test signal,the direct signal synthesizer 274 is preferably controlled by the fieldprogrammable gate array 272 and processor 270 to generate a standardreflectometry signal according to any technique well known in the art.Such techniques include, but are not limited to, time domainreflectometry, frequency domain reflectometry and techniques involvingthe generation of other wideband signals such as pseudo random noise.These techniques can be used to determine the position and nature ofchanges in the characteristic impedance of the pair-to-pair channelwithin the patch cables 210. Such changes in impedance typically resultfrom end-connections, cable connections, faults and otherdiscontinuities.

Once generated, the single-ended voltage signal from the direct signalsynthesiser 274 is multiplexed by the multiplexer 275, amplified by theamplifier 226, and is then converted into a differential voltage signaland coupled to the patch cable 210 by the converting means 224, theelectrical isolating means 222 and the first pair of electrodes 220 a,220 b of the coupling unit 220 in same way as described above withreference to FIGS. 5 and 6, i.e. such that the voltage signal propagatesbetween at least two of the twisted pairs in the patch cable 210.

When a voltage signal which propagates between the twisted pairspropagates along the cable 210 to the coupling unit 220, the voltagesignal is received and converted into a single-ended voltage signal bythe second pair of electrodes 240 a, 240 b, the converting means 242 andthe electrical isolating means 244 of the coupling unit 220 in the sameway as described above with reference to FIGS. 5 and 6. The single-endedvoltage signal is then amplified by the amplifier 246, demultiplexed bythe multiplexer 275, demodulated by the multiplier 276 and the low passfilter and amplifier 278 and is then passed to the analogue to digitalconverter 279 where it is converted into a digital signal. A final stageof demodulation is performed by the field programmable gate array 272and the signal is then passed to the processor 270.

In the case of a data signal being received by the coupling unit 220,the processor 270 can identify the coupling unit 220 which coupled thedata signal to the patch cable 210 by retrieving the address datacontained therein. This information can then be supplied to the controlunit 292, which can use the information to determine interconnections bythe patch cables 210 between the network ports in the local area networkshown in FIG. 7.

The control unit 292 may be programmed to control the signal units 290to determine the interconnections between the network ports in the localarea network according to any suitable scheme. For example, the controlunit 292 may control each signal unit 290 to be in either a transmitmode or a receive mode. In the transmit mode, each coupling unit 220 inthe signal unit 290 couples a data signal containing address data toidentify the coupling unit 220. In the receive mode, each coupling unit220 in the signal unit 290 waits to see if it receives a data signalfrom a coupling unit 220 of a signal unit 290 in transmission mode.Based on address data retrieved by the processors 270 of signal units290 in the receive mode, the control unit 292 can determine whichnetwork ports are interconnected by patch cables 210 in the local areanetwork. However, as would be appreciated by the skilled person, thereare many possible schemes that could be used to determine theinterconnections within the local area network.

An advantage of using the network apparatus shown in FIG. 7 to determinethe interconnections between the network ports in a local area networkis that it can be retrofitted to an existing local area network withoutrequiring substantial modification of that local area network. Inparticular, it is not necessary to install specially adapted patchcables or patch panels and the transmission of signals within thetwisted pairs of each patch cable need not be interrupted.

In the case of a test signal being received by the coupling unit 220,the processor 270 can determine a state of the cable 210, e.g. bycomparing the received signal with a reference signal, as is known inthe art in standard reflectometry techniques. Such techniques areexplained, for example, in International Patent Application publicationnumber WO2005/109015. In one example, a reference signal is recordedwhen the network is known to be in a good operation. A signal measuredby a coupling unit can subsequently be compared with the referencesignal to determine whether a fault condition has occurred. The controlunit 292 may be suitably arranged to control the signal units 290 tocontinually or intermittently determine the state of the patch cables210. Reflectometry techniques generally involve the receiving ofreflected signals, and therefore the test signal may be coupled to andfrom a patch cable 210 by the same coupling unit 220.

Local area networks vary widely in scale and complexity. For example,the local area network shown in FIG. 7 may include “cross-connect” patchpanels located between the first patch panels 280 a and the second patchpanels 280 b. The network apparatus may therefore be extended and/ormodified according to the local area network with which it is to beused.

In the above described network apparatus, each coupling unit 220 housesa first pair of electrodes 220 a, 220 b for coupling a voltage signal toa patch cable 210 and a second pair of electrodes 240 a, 240 b forcoupling a voltage signal from a patch cable 210. In other possiblearrangements, each coupling unit 220 may be provided only with a singlepair of electrodes. This may reduce costs. The single pair of electrodesmay, for example, be arranged only to couple a voltage signal to a patchcable or only to couple a voltage signal from a patch cable.Alternatively, the single pair of electrodes may be arranged to bothcouple a voltage signal to a patch cable and to couple a voltage signalfrom a patch cable.

In some arrangements, the signal units 290 may be provided with couplingunits 220 arranged only to couple a voltage signal to a patch cable oronly to couple a voltage signal from a patch cable. However, it ispreferred for each signal units 290 to have coupling units 220 able tocouple a signal to and from a patch cable 210, e.g. as shown in FIG. 9,since this allows the signal units 290 to communicate in two directions.

Also, coupling units 220 arranged to couple a signal to and from a patchcable 210 may be particularly useful for reflectometry, as they can bothsend test signals and receive the reflected test signals. The circuitryrequired for reflectometry may be expensive. Therefore, it may bepreferable to provide only a small number of signal units 290 which havethe circuitry required for reflectometry positioned on the server linesof a local area network, as generally only the state of live cables in alocal area network need to be monitored.

It is typical for a patch panel to have twenty-four network ports.Therefore, each signal unit 290 may therefore have twenty-four couplingunits 220. However, the number of coupling units 220 within each signalunit 290 may be modified according to the patch panels with which thesignal unit 290 is to be used.

Although the data signals described above contain address data, thesignal units 290 could equally be used to send other types of datathrough the patch cables 210. Accordingly, the signal units 290 could beused to provide communications channels which are additional to thosecontained within each twisted pair in the patch cables 210.

To help in the understanding of the present invention, the followingexamples are presented.

EXAMPLE 1

To investigate the nature of the propagation of a voltage signalpropagating between twisted pairs in a cable including a plurality oftwisted pairs, an unshielded twisted pair cable including four twistedpairs was modelled.

Three dimensional finite element models were used to determine allcapacitance and inductance parameters of the unshielded twisted paircable. These capacitance and inductance values were fed into anequivalent circuit model of the unshielded twisted pair cable.

FIG. 10 shows the equivalent circuit model of a short elemental sectionof an unshielded twisted pair cable 310 including eight conductors 1 to8 which have been twisted together in pairs to form four twisted pairs1-2, 3-4, 5-6, 7-8. Such elemental sections can be connected end to endto model any length of an unshielded twisted pair cable including fourtwisted pairs. In addition, transmitters and receivers can be added tocomplete the system, which can then be analysed using a circuitmodelling tool such as SPICE (“Simulation Program with IntegratedCircuit Emphasis”). Such modelling tools are well known by personsskilled in the art.

FIG. 11 shows a twisted pair cable 310 that was modelled using theequivalent circuit model of FIG. 10, along with two electrodes 320 a and320 b for coupling a voltage signal to the twisted pair cable 310 sothat the voltage signal propagates between at least two of the twistedpairs in the cable, e.g. as described previously. As shown in FIG. 11,the electrodes 320 a and 320 b are adjacent the twisted pairs 1-2 and5-6 and are therefore arranged to couple a voltage signal to the cableso that the voltage signal propagates between at least the twisted pairs1-2 and 5-6.

As illustrated by FIG. 11, there are six pair-to-pair combinationswithin the twisted pair cable 310, namely:

Pairs 1-2 and 3-4

Pairs 1-2 and 5-6

Pairs 1-2 and 7-8

Pairs 3-4 and 5-6

Pairs 3-4 and 7-8

Pairs 5-6 and 7-8

FIGS. 12 a-c show the simulated voltage signal propagation along thetwisted pair cable 310 shown in FIG. 11. FIG. 12 a shows the simulatedvoltage signal propagation between pairs 1-2 and 5-6. FIG. 12 b showsthe simulated voltage signal propagation between pairs 1-2 and 3-4. FIG.12 c shows the simulated voltage signal propagation between pairs 3-4and 7-8. Within each pair-to-pair combination, individual voltagesignals are labelled i-vi. In all cases, the delay between each voltagesignal was 7.5 ns.

The voltage signals between pairs 1-2 and 7-8, pairs 3-4 and 5-6, andpairs 5-6 and 7-8, are not shown for brevity but, owing to symmetry,would be the same as or very similar to the voltage signals betweenpairs 1-2 and 3-4 shown in FIG. 12 b.

As can be seen from FIGS. 12 a and 12 b, the voltage signals betweenpairs 1-2 and 5-6 (which are adjacent to the electrodes 320 a and 320 bof the transmitter) are approximately twice as strong as the voltagesignals between pairs 1-2 and 3-4. As can be seen from FIG. 12 c, thevoltage signals between pairs 3-4 and 7-8 are much smaller, but increasewith distance along the twisted pair cable 310. This shows that thevoltage signals propagating between twisted pairs 1-2 and 5-6 haveleaked on to other pair-to-pair channels such as the channel defined bytwisted pairs 3-4 and 7-8.

EXAMPLE 2

FIG. 13 shows a test apparatus for coupling a voltage signal whichpropagates between twisted pairs to and from a twisted pair cable, whichin this case is an unshielded twisted pair patch cable 410.

The apparatus of FIG. 13 has a first coupling unit 420 for coupling avoltage signal to the patch cable 410, a second coupling unit 440 forcoupling a voltage signal from the patch cable 410, a computer 492 and avector network analyser 499. The vector network analyser 499 iscontrolled by the computer 492 and is connected to the first couplingunit 420 and the second coupling unit 440 by two signal leads 432. Thecomputer 492 is programmed with signal processing and analysis softwarefor controlling the vector network analyser to apply a frequency sweep,acquiring the resultant in-phase and quadrature signal components, andanalysing this data with an inverse fourier transform to produce timedomain reflectometry plots.

The first coupling unit 420 includes two electrodes 420 a and 420 b, abalun 422 and a choke 424, in an arrangement corresponding to that shownin FIG. 6. The first coupling unit 420 is connected to the vectornetwork analyser 499 via one of the signal leads 432. The balun 422 is a50:100 Ohms device.

The second coupling unit 440 includes two electrodes 440 a and 440 b, abalun 442 and a choke 444, in an arrangement corresponding to that shownin FIG. 6. The second coupling unit 440 is connected to the vectornetwork analyser 499 by the other of the signal leads 432. The balun 422is a 50:100 Ohms device.

FIGS. 14, 15 and 16 are time domain reflectometry plots produced usingthe apparatus of FIG. 13. To produce these plots, time domainreflectometry was performed under a variety of different conditions. Inall examples, the patch cable 410 was a 10 metre unshielded twisted paircable containing four twisted pairs and terminated with an RJ45 typeconnector.

To produce the plot shown in FIG. 14 a the patch cable 410 was connectedto an (unterminated) additional 3 metre unshielded twisted pair patchcable. In FIG. 14 a the reflections caused by the RJ45 type connectorand the end of the additional patch cable are both clearly visible andare marked X and Y1 respectively. To produce the plot shown in FIG. 14b, the patch cable 410 was connected to an (unterminated) additional 10metre unshielded twisted pair patch cable. In FIG. 14 b, the reflectionscaused by the RJ45 type connector and the end of the additional patchcable are both clearly visible and are marked X and Y2 respectively.

To produce the plots shown in FIGS. 15 a and 15 b, the cable 410 wasconnected to an additional 10 metre unshielded twisted pair patch cable.For FIG. 15 a, the additional patch cable was unterminated, whereas forFIG. 15 b, each of the four twisted pairs in the additional patch cablewas terminated with a 100 Ohm termination. The value of 100 Ohm waschosen because it matched the characteristic impedance of each twistedpair in the additional patch cable. These terminations should thereforeattenuate differential voltage signals which propagate within, ratherthan between, the twisted pairs in the twisted pair cables.

By comparing the plots of FIGS. 15 a and 15 b, it can be seen that thetime domain reflectometry plot is unaffected by the presence of the 100Ohm resistor terminations. This indicates that the voltage signalcoupled to the twisted pair cable 410 by the first coupling unit 420does not include, at least not to a significant extent, differentialvoltage signals which propagate within the twisted pairs of the twistedpair cable 410, because the 100 Ohm terminations has a negligible effecton the measured signal.

To produce the plots shown in FIGS. 16 a and 16 b, the cable 410 wasconnected to and terminated by an additional 10 metre unshielded twistedpair patch cable. For FIG. 16 a, the additional patch cable wasunterminated, whereas for FIG. 16 b, each of the four twisted pairs inthe additional patch cable was terminated by a matched inter-pairtermination resistor network, which is shown in FIG. 17. By comparingthe plots of FIGS. 16 a and 16 b, it can be seen that the reflectionscaused by the RJ45 type connector and the additional patch cable aresignificantly reduced in the case of FIG. 16 b. This indicates that thevoltage signal from the first coupling unit is largely propagatingbetween at least two twisted pairs in the patch cables.

One of ordinary skill after reading the foregoing description will beable to affect various changes, alterations, and subtractions ofequivalents without departing from the broad concepts disclosed. It istherefore intended that the scope of the patent granted hereon belimited only by the appended claims, as interpreted with reference tothe description and drawings, and not by limitation of the embodimentsdescribed herein.

The following statements provide examples of general expressions of thedisclosure herein.

-   -   A. A coupling unit for coupling a voltage signal to and/or from        a cable including a plurality of twisted pairs, the coupling        unit having:        -   a first electrode and a second electrode arranged to produce            an electric field therebetween to couple a voltage signal to            the cable by non-contact coupling with the twisted pairs so            that the voltage signal propagates along the cable between            at least two of the twisted pairs and/or arranged to receive            a voltage signal which has propagated along the cable            between at least two of the twisted pairs by non-contact            coupling with at least two of the twisted pairs between            which the voltage signal has propagated; and        -   a voltage signal coupling means for coupling a voltage            signal generated by a voltage signal generator to the            electrodes and/or for coupling a voltage signal received by            the electrodes to a voltage signal processor;        -   wherein the voltage signal coupling means includes            electrical isolation means arranged to electrically isolate            the electrodes of the coupling unit from the voltage signal            generator and/or the voltage signal processor.    -   B. A coupling unit according to statement A wherein the        electrical isolation means includes a balun.    -   C. A coupling unit according to statement A or B wherein the        coupling unit includes shielding for shielding the electrodes        from an external electromagnetic field.    -   D. A coupling unit according to any one of the previous        statements wherein the voltage signal coupling means includes a        converting means for converting a single-ended voltage signal        from a voltage signal generator into a differential voltage        signal to be coupled to the electrodes and/or for converting a        differential voltage signal from the electrodes into a        single-ended voltage signal to be coupled to a voltage signal        processor.    -   E. A coupling unit according to statement D wherein the        electrical isolating means electrically isolates the electrodes        from the converting means.    -   F. A coupling unit according to statement D or E wherein the        converting means includes a choke.    -   G. A coupling unit according to any one of the previous        statements wherein the coupling unit has housing in which the        electrodes, and optionally any one or more of the voltage signal        coupling means, the electrical isolating means, the shielding        and the converting means, is housed.    -   H. A coupling unit according to statement G wherein the        electrodes are spaced apart by the housing to allow the cable to        be received therebetween.    -   I. A coupling unit according to statement G or H wherein the        electrodes are spaced apart by the housing so as to be located        on directly opposite sides of the cable when the cable is        received therebetween.    -   J. A coupling unit according to any one of statements G to I        wherein the housing is arranged to be clipped on to the cable.    -   K. A coupling unit according to statement J wherein the        electrodes are located in the housing so as to contact an outer        surface of the cable if the housing is clipped onto the cable.    -   L. A coupling unit according to any one of the previous        statements wherein each electrode includes a respective contact        surface for contacting an outer surface of the cable.    -   M. A coupling unit according to statement L wherein each contact        surface substantially conforms in shape to an outer surface of        the cable.    -   N. A coupling unit according to any one of the previous        statements wherein the coupling unit includes a third electrode        and a fourth electrode, which are spaced apart from the first        and second electrodes and arranged to produce an electric field        therebetween to couple the voltage signal to the cable by        non-contact coupling with the twisted pairs so that the voltage        signal propagates between at least two of the twisted pairs        and/or arranged to receive a voltage signal which has propagated        along the cable between at least two of the twisted pairs by        non-contact coupling with at least two of the twisted pairs        between which the voltage signal has propagated.    -   O. A coupling unit according to any one of the previous        statements wherein:        -   the first electrode and the second electrode are arranged to            produce an electric field therebetween to couple a voltage            signal to the cable by non-contact coupling with the twisted            pairs so that the voltage signal propagates along the cable            between at least two of the twisted pairs;        -   the voltage signal coupling means is for coupling a voltage            signal generated by a voltage signal generator to the            electrodes; and        -   the electrical isolation means is arranged to electrically            isolate the electrodes of the coupling unit from the voltage            signal generator.    -   P. A coupling unit according to statement O having:        -   a first additional electrode and a second additional            electrode arranged to receive a voltage signal which has            propagated along the cable between at least two of the            twisted pairs by non-contact coupling with at least two of            the twisted pairs between which the voltage signal has            propagated; and        -   a voltage signal coupling means for coupling a voltage            signal received by the additional electrodes to a voltage            signal processor;        -   wherein the voltage signal coupling means includes            electrical isolation means arranged to electrically isolate            the additional electrodes of the coupling unit from the            voltage signal processor.    -   Q. A coupling unit according to any one of statements A to N        wherein:        -   the first electrode and the second electrode are arranged to            receive a voltage signal which has propagated along the            cable between at least two of the twisted pairs by            non-contact coupling with at least two of the twisted pairs            between which the voltage signal has propagated;        -   the voltage signal coupling means is for coupling a voltage            signal received by the electrodes to a voltage signal            processor; and        -   the electrical isolation means is arranged to electrically            isolate the electrodes of the coupling unit from the voltage            signal processor.    -   R. An apparatus having:        -   a coupling unit according to any one of the previous            statements; and        -   a voltage signal generator;        -   wherein the voltage signal coupling means couples the            voltage signal generator to the electrodes of the coupling            unit.    -   S. An apparatus according to statement R wherein the voltage        signal generator is arranged to generate a data signal which        contains data.    -   T. An apparatus according to statement S wherein the data signal        contains address data to identify the coupling unit.    -   U. An apparatus according to any one of statements R to T        wherein the voltage signal generator is arranged to generate a        test signal for determining a state of the cable.    -   V. An apparatus according to statement U wherein the test signal        is suitable for performing time domain reflectometry or        frequency domain reflectometry.    -   W. An apparatus having:        -   a coupling unit according to any one of the previous            statements; and        -   a voltage signal processor;        -   wherein the voltage signal coupling means couples the            voltage signal processor to the electrodes of the coupling            unit.    -   X. An apparatus according to statement W wherein the signal        processor includes a data retrieving means for retrieving data        from a data signal received by the receiver.    -   Y. An apparatus according to statement W or X wherein the signal        processor is arranged to determine a state of a cable based on a        test signal received by the receiver.    -   Z. An apparatus having:        -   a cable including a plurality of twisted pairs;        -   a first coupling unit for coupling a voltage signal to the            cable as set out in any previous statement;        -   a second coupling unit for coupling a voltage signal from            the cable as set out in any previous statement.    -   ZA. An apparatus according to statement Z wherein the cable is        an unshielded twisted pair cable.    -   ZB. An apparatus for coupling a signal to and from a cable        including a plurality of twisted pairs, the apparatus having:        -   a first coupling unit arranged to couple a signal to the            cable by non-contact coupling with the twisted pairs so that            the signal propagates along the cable between at least two            of the twisted pairs; and        -   a second coupling unit arranged to receive the signal which            has propagated along the cable by non-contact coupling with            at least two of the twisted pairs between which the signal            has propagated.    -   ZC. An apparatus according to statement ZB wherein the signal is        a voltage signal.    -   ZD. An apparatus according to statement ZC wherein the first        coupling unit is arranged such that the voltage signal which        propagates between at least two of the twisted pairs has an        average voltage which corresponds to the average voltage in the        conductors which form the twisted pairs.    -   ZE. An apparatus according to statement ZD wherein the first        coupling unit is arranged such that the voltage signal which        propagates between at least two of the twisted pairs has an        average voltage which tracks the average voltage in the        conductors which form the twisted pairs over time.    -   ZF. An apparatus according to any one of statements ZC to ZE        wherein the first coupling unit has a first electrode and a        second electrode arranged to produce an electric field        therebetween to couple a voltage signal to the cable by        non-contact coupling with the twisted pairs so that the voltage        signal propagates along the cable between at least two of the        twisted pairs.    -   ZG. An apparatus according to statement ZF wherein the first        coupling unit is a coupling unit for coupling a voltage signal        to a cable including a plurality of twisted pairs as set out in        any one of statements A to ZA.    -   ZH. An apparatus according to statement ZG wherein the        electrical isolating means is omitted.    -   ZI. An apparatus according to any one of statements ZC to ZH        wherein the second coupling unit has a first electrode and a        second electrode arranged to receive a voltage signal which has        propagated along the cable between at least two of the twisted        pairs by non-contact coupling with at least two of the twisted        pairs between which the voltage signal has propagated.    -   ZJ. An apparatus according to statement ZI wherein the second        coupling unit is a coupling unit for coupling a voltage signal        from a cable including a plurality of twisted pairs as set out        in any one of statements A to ZB.    -   ZK. An apparatus according to statement ZJ wherein the        electrical isolating means is omitted.    -   ZL. A first coupling unit for coupling a signal to a cable        including a plurality of twisted pairs as set out in any one of        statements ZB to ZK.    -   ZM. A second coupling unit for coupling a signal from a cable        including a plurality of twisted pairs as set out in any one of        statements ZB to ZK.    -   ZN. An apparatus having:        -   one or more first coupling units for coupling a signal to a            cable including a plurality of twisted pairs so that the            signal propagates along the cable between at least two of            the twisted pairs;        -   one or more second coupling units for coupling a signal            which has propagated between at least two twisted pairs from            a cable including a plurality of twisted pairs; and        -   an interconnection determining means arranged to determine            one or more interconnections between any one of the first            coupling units and any one of the second coupling units by            one or more cables including a plurality of twisted pairs;        -   wherein each of the one or more first coupling units is as            set out in any one of statements A to ZA or ZL, and/or each            of the one or more second coupling units is as set out in            any one of statements A to ZA or ZM.    -   ZO. An apparatus according to statement ZN wherein the        interconnection determining means is arranged to determine an        interconnection between one of the first coupling units and one        of the second coupling units by determining if the second        coupling unit couples a signal from a cable which has been        coupled to the cable by the first coupling unit.    -   ZP. An apparatus according to statement ZN or ZO wherein each of        the one or more first coupling units is arranged to couple to        the cable a data signal containing address data to identify the        first coupling unit.    -   ZQ. An apparatus according to any one of statements ZN to ZP        wherein each of the one or more first coupling units and each of        the one or more second coupling units is associated with a        respective network port in a local area network, and the        interconnection determining means is arranged to determine one        or more interconnections between the network ports by one or        more cables including a plurality of twisted pairs.    -   ZR. An apparatus according to any one of statements ZN to ZQ        wherein the network apparatus includes one or more signal units,        each signal unit including:        -   a plurality of the first coupling units; and/or        -   a plurality of the second coupling units.    -   ZS. A signal unit as set out in statement ZR.    -   ZT. A method of coupling a voltage signal to and/or from a cable        including a plurality of twisted pairs, the method including:        -   producing an electric field to couple a voltage signal to            the cable by non-contact coupling with the twisted pairs so            that the voltage signal propagates along the cable between            at least two of the twisted pairs; and/or        -   receiving a voltage signal which has propagated along the            cable between at least two of the twisted pairs by            non-contact coupling with at least two of the twisted pairs            between which the voltage signal has propagated.    -   ZU. A method of coupling a signal to and from a cable including        a plurality of twisted pairs, the method including:        -   coupling a signal to the cable by non-contact coupling with            the twisted pairs so that the signal propagates along the            cable between at least two of the twisted pairs; and        -   receiving the signal which has propagated along the cable by            non-contact coupling with at least two of the twisted pairs            between which the signal has propagated.    -   ZV. A coupling unit, apparatus or signal unit substantially as        herein described, with reference to and as shown in FIGS. 4 to        6, FIGS. 7 to 9, FIGS. 10 to 12 or FIGS. 13 to 17.

1. An apparatus for coupling a signal to and from a cable including aplurality of twisted pairs, the apparatus having: a first coupling unitarranged to couple a signal to the cable by non-contact coupling withthe twisted pairs so that the signal propagates along the cable betweenat least two of the twisted pairs; and a second coupling unit arrangedto receive the signal which has propagated along the cable bynon-contact coupling with at least two of the twisted pairs betweenwhich the signal has propagated.
 2. An apparatus according to claim 1wherein the signal is a voltage signal.
 3. An apparatus according toclaim 2 wherein the first coupling unit is arranged such that thevoltage signal which propagates between at least two of the twistedpairs has an average voltage which corresponds to the average voltage inthe conductors which form the twisted pairs.
 4. An apparatus accordingto claim 3 wherein the first coupling unit is arranged such that thevoltage signal which propagates between at least two of the twistedpairs has an average voltage which tracks the average voltage in theconductors which form the twisted pairs over time.
 5. An apparatusaccording to claim 2 wherein the first coupling unit has a firstelectrode and a second electrode arranged to produce an electric fieldtherebetween to couple a voltage signal to the cable by non-contactcoupling with the twisted pairs so that the voltage signal propagatesalong the cable between at least two of the twisted pairs.
 6. Anapparatus according to claim 5 wherein the first coupling unit is acoupling unit for coupling a voltage signal to a cable including aplurality of twisted pairs, the coupling unit having: a first electrodeand a second electrode arranged to produce an electric fieldtherebetween to couple a voltage signal to the cable by non-contactcoupling with the twisted pairs so that the voltage signal propagatesalong the cable between at least two of the twisted pairs; and a voltagesignal coupling means for coupling a voltage signal generated by avoltage signal generator to the electrodes; wherein the voltage signalcoupling means includes electrical isolation means arranged toelectrically isolate the electrodes of the coupling unit from thevoltage signal generator.
 7. An apparatus according to claim 6 whereinthe electrical isolating means is omitted.
 8. An apparatus according toclaim 2 wherein the second coupling unit has a first electrode and asecond electrode arranged to receive a voltage signal which haspropagated along the cable between at least two of the twisted pairs bynon-contact coupling with at least two of the twisted pairs betweenwhich the voltage signal has propagated.
 9. An apparatus according toclaim 8 wherein the second coupling unit is a coupling unit for couplinga voltage signal from a cable including a plurality of twisted pairs,the coupling unit having: a first electrode and a second electrodearranged to receive a voltage signal which has propagated along thecable between at least two of the twisted pairs by non-contact couplingwith at least two of the twisted pairs between which the voltage signalhas propagated; and a voltage signal coupling means for coupling avoltage signal received by the electrodes to a voltage signal processor;wherein the voltage signal coupling means includes electrical isolationmeans arranged to electrically isolate the electrodes of the couplingunit from the voltage signal processor.
 10. An apparatus according toclaim 9 wherein the electrical isolating means is omitted.
 11. Anapparatus according to claim 1 wherein the first coupling unit and thesecond coupling unit are the same coupling unit.
 12. A method ofcoupling a signal to and from a cable including a plurality of twistedpairs, the method including: coupling a signal to the cable bynon-contact coupling with the twisted pairs so that the signalpropagates along the cable between at least two of the twisted pairs;and receiving the signal which has propagated along the cable bynon-contact coupling with at least two of the twisted pairs betweenwhich the signal has propagated.
 13. A method of coupling a voltagesignal to and/or from a cable including a plurality of twisted pairs,the method including: producing an electric field to couple a voltagesignal to the cable by non-contact coupling with the twisted pairs sothat the voltage signal propagates along the cable between at least twoof the twisted pairs; and/or receiving a voltage signal which haspropagated along the cable between at least two of the twisted pairs bynon-contact coupling with at least two of the twisted pairs betweenwhich the voltage signal has propagated.
 14. An apparatus having: one ormore first coupling units for coupling a signal to a cable including aplurality of twisted pairs so that the signal propagates along the cablebetween at least two of the twisted pairs; one or more second couplingunits for coupling a signal which has propagated between at least twotwisted pairs from a cable including a plurality of twisted pairs; andan interconnection determining means arranged to determine one or moreinterconnections between any one of the first coupling units and any oneof the second coupling units by one or more cables including a pluralityof twisted pairs.
 15. An apparatus according to claim 14 wherein theinterconnection determining means is arranged to determine aninterconnection between one of the first coupling units and one of thesecond coupling units by determining if the second coupling unit couplesa signal from a cable which has been coupled to the cable by the firstcoupling unit.
 16. An apparatus according to claim 14 wherein each ofthe one or more first coupling units is arranged to couple to the cablea data signal containing address data to identify the first couplingunit.
 17. An apparatus according to claim 14 wherein each of the one ormore first coupling units and each of the one or more second couplingunits is associated with a respective network port in a local areanetwork, and the interconnection determining means is arranged todetermine one or more interconnections between the network ports by oneor more cables including a plurality of twisted pairs.
 18. An apparatusaccording to claim 14 wherein the network apparatus includes one or moresignal units, each signal unit including: a plurality of the firstcoupling units; and/or a plurality of the second coupling units.